An LCD has the advantages of portability, low power consumption, and low radiation, and has been widely used in various portable information products such as notebooks, personal digital assistants (PDAs), video cameras and the like. Furthermore, the LCD is considered by many to have the potential to completely replace CRT (cathode ray tube) monitors and televisions.
Usually, an LCD needs an external power supply for providing operating power. When the LCD operates, much electric charge is stored therein. When the LCD is powered off, electric charge stored therein is not discharged quickly. This makes the voltage at the external power supply connection drop slowly. As a result, a gate driving circuit and a data driving circuit that drive the LCD operate incorrectly, thereby producing a residual image on the LCD.
FIG. 4 is a schematic circuit diagram of a typical LCD. The LCD 10 includes an LCD panel 14 and a driving circuit (not labeled). The driving circuit includes a gate driving integrated circuit (IC) 12, a data driving IC 13, a primary control circuit board 11, and a flexible printed circuit board (FPCB) 15. The gate driving IC 12 and the data driving IC 13 are respectively formed on two adjacent sides of the LCD panel 14 by chip on glass (COG) technology. The FPCB 15 is connected between the LCD panel 14 and the primary control circuit board 11. The gate driving IC 12 scans the LCD panel 14. The data driving IC 13 provides a plurality of gradation voltages to the LCD panel 14 when the LCD panel 14 is scanned.
The primary control circuit board 11 includes a driving control circuit 111, a power supply circuit 112 and a voltage detecting circuit 113. The power supply circuit 112 directly provides an operation voltage (not labeled) to the driving control circuit 111, and respectively provides two operation voltages V2, V1 to the gate driving IC 12 and the data driving IC 13 via the FPCB 15.
The voltage detecting circuit 113 generates an all-scanning signal “Xon” when the operation voltage V2 falls below a predetermined threshold voltage. Then, the voltage detecting circuit 113 transmits the all-scanning signal “Xon” to the gate driving IC 12 through a conducting lead (not labelled) configured on the FPCB 15. As the gate driving IC 12 receives the all-scanning signal “Xon”, the gate driving IC turns on all of TFTs (thin film transistors) of the LCD panel 14. Thus the electric charge stored in the TFTs can be discharged quickly. As a result, the residual image on the LCD 10 does not appear when the LCD 10 is power off.
However, because the primary control circuit board 11 includes the voltage detecting circuit 113, the volume occupied by the primary control circuit board 11 is large. Furthermore, because the voltage detecting circuit 113 is configured on the primary control circuit board 11, the FPCB 15 needs a special conducting lead formed thereon for transmitting the all-scanning signal “Xon” from the voltage detecting circuit 113 to the gate driving IC 12.
What is needed, therefore, is a driving circuit of an LCD that can overcome the above-described deficiencies.